1. Field of the Invention
The present invention relates to a device for crossing optical beams, in particular in an integrated optical circuit.
2. Description of the Related Art
In optical telecommunication systems, information is typically coded in short optical pulses by suitable optical sources, such as light-emitting diodes (LEDs) or semiconductor lasers, which pulses are transmitted along an optical-fibre network and received by photodetectors. Many different signals can be transmitted using a single wavelength of light by interweaving the pulses from different sources, a technique known as time-division multiplexing (TDM).
A simple way of increasing the amount of data that can be transmitted by a single optical fibre is to make the incoming electronic bits as short as possible. Current optical systems have achieved data rates up to 40 gigabits per second.
Recently, transmission capacity has been increased by dense wavelength division multiplexing (DWDM), which requires a very stable emitting laser and, at the receiver, very narrow linewidth filters and optical switches for separating individual wavelength channels and routing them to the appropriate destinations. Due to the large number of individual components in a DWDM system, integrated optical circuits have been developed. Integrated optical circuits may be either monolithic or hybrid and comprise active and passive components, typically realized on a semiconductor or dielectric substrate, used for coupling between optoelectronic devices and providing signal processing functions.
Since integrated circuits provide for designing of integrated components and waveguides substantially in a two-dimensional structure, one problem in integrated optical circuits is crossing of different waveguides. This problem occurs for example in case of optical switching, where a plurality of optical inputs are directed to a plurality of optical outputs, and crossing is necessary in order for each input to connect to every output.
This problem is addressed for example in U.S. Pat. No. 6,198,860, wherein it is observed that current integration in the wavelength scale requires intersection of waveguides, and not simply pass of the waveguide over the other, since any realizing three-dimensional structures adds considerable manufacturing difficulty. A main problem with waveguide crossing is crosstalk, i.e. undesired redirection of part of a signal conveyed in a waveguide into a crossing waveguide. When two crossing waveguides are conveying respective signals, crosstalk produces an interference between the two signal.
U.S. Pat. No. 6,198,860 provides a solution to this problem by providing, at the intersection between two waveguides, a photonic crystal resonator.
Photonic crystals are dielectric structures having a periodic variation (or modulation) of the dielectric constant along one, two or three directions of space (and the crystal is therefore referred to as a 1-D, 2-D or 3-D photonic crystal). A 2-D photonic crystal typically comprises a piece of dielectric material (for example an optically thin slab) wherein a periodic array of regions of different refractive index is realized. These regions may be defined, for example, by cylinders filled of air or other predetermined substances or materials.
Photonic crystals have band gaps that restrict the propagation of light in certain frequency ranges. Their discovery in recent years has caused a rethinking of conventional methods for manipulating light, and has led to proposals for many novel optical devices. In particular, it has been shown that a linear defect in a photonic crystal (i.e. a break in the periodicity of the crystal along one line) allows light with wavelength within the bandgap to be guided, by relying on the band gap restriction instead of index confinement to prevent light from escaping. Similarly, a defect at a single location (a point defect) creates a resonant cavity, which traps light in a small region.
The waveguide crossing design proposed in U.S. Pat. No. 6,198,860 makes uses of both of these phenomena, since light is guided to the crossing region through linear defects, while the crossing region contains a single defect defining a resonant cavity. FIG. 1 shows one possible embodiments as depicted in U.S. Pat. No. 6,198,860. A photonic crystal 10 has a first and a second waveguide 11, 12 perpendicular to each other, defined by respective linear defects, and a resonant crossing region 13, formed by a single defect. The single defect is defined by a dielectric rod of larger radius than the other rods of the crystal.
As far as the guided mode is concerned, the situation is described by simple one-dimensional resonant tunneling, wherein the crossing waveguide is effectively invisible. In particular, when a guided mode is incident upon the crossing from one of the waveguides, it can only couple to the resonant state that is symmetric with respect to the axis of that waveguide. The other resonant state, which is anti-symmetric, is orthogonal to that guided mode by symmetry. Correspondingly, the symmetric resonant state can only decay into the input and output waveguides, since this state is orthogonal to the modes in the crossing waveguide.
This structure allows optical modes to propagate with 100% transmission (throughput) from an input waveguide to the output waveguide on the opposite side of a crossing, with no reflection and with 0% transmission (crosstalk) to the crossing waveguide.
The Applicant, tackling the problem of avoiding crosstalk in intersecting optical waveguides, has searched for alternative solutions.
In particular, the Applicant has considered possible different X-crossing structures using photonic crystals.
Besides development of photonic crystals devices having defects, behaviour of light in photonic crystals having regular periodicity (herein below also referred to as “regular photonic crystals”), has been investigated. For the purposes of the present invention, with “photonic crystals having regular periodicity” it is intended a photonic crystal wherein the characteristics of its periodic array do not vary at least in a region thereof of intended light propagation.
The article of P. Etchegoin and R. T. Phillips, “Photon focusing, internal diffraction, and surface states in periodic dielectric structures”, Physical Review B, Volume 53, Number 19, 15 May 1996-1, takes advantage of some analogies between electrons in semiconductors and electromagnetic waves in periodic dielectric structures for providing a method for calculation the band structure of a 2-D periodic dielectric structure. Moreover, this article deals with the phenomenon of photon focusing emitted by a source point in these structure, in analogy with the phenomenon of acoustic phonon focusing, showing what shape shall have the kx-ky diagram of the wave vector k to have focusing of light along predetermined directions.
The Applicant observes that, besides having being studied only at theoretical level, applications of photon focusing would be of limited use in integrated optics.
The article of Marko Lonar, Jelena Vukovi and Axel Scherer, “Three-dimensional analysis of dispersion properties of planar photonic crystals”, Proceedings of PECS III conference (June 2001), St. Andrew's, Scotland. shows that a planar (i.e. 2-D) photonic crystal may have, under certain conditions, a self-collimation effect in the second energy band (i.e. the energy band over the fundamental band). As disclosed in the article, these conditions determine a negative group velocity.
The Applicant observes that, for the time being, no practical applications have been shown of a beam of light (although collimated) having a negative group velocity.
The Applicant has further investigated the light propagation properties of regular photonic crystals and considered the possibility of using such structures for X-crossing purposes.